What is it?
Prototyping is the process of making the simulation or sample version of the real product in order to test it and get feedback.
How can I use it?
If you are at the early stages of your process, paper and pen can be enough. You can draw the screens on the different pages and manually operate the flow by user's action. In latter stages you can benefit from the different digital tools that automatizes the flow between the screens.
When can I use it?
You can use it in different phases of the design process. In early phase for getting quick feedback, in the latter phases usability testing or presenting the design to the internal stakeholders.